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Use US Businesses/Quick Search to look up known company and see list of potential competitors. Or use US Businesses/Custom Search to create list of companies in certain geographic area in a certain line of business.
Use Get Company Info tab in lower right or Company Dossier lookup to Create a Company List Includes info from various sources, including Key Competitors (from Hoover's).
Find information about known companies or create lists of companies meeting certain criteria related to factors such as industry, geographic location, and size.